Prime Minister John Key says Police Associate president Greg O'Connor should present any evidence he has that MPs are increasingly interfering in police work.

In the wake of Maurice Williamson's resignation as a minister for interfering in a police case, Mr O'Connor said it was increasingly hard for the police to be free of political interference because of the calls they get from public servants and politicians for information about cases they were working on.

Mr Key said he had no evidence to support Mr O'Connor's claims.

"If he wants to come forward with that, he absolutely should do so," he told reporters at the National Party northern regional conference.

Mr Key said Mr Williamson's call to the police - in the middle of an investigation into a domestic violence case - put the police in an unacceptable position.

"As result of that phone call the police undertook a review that would not otherwise havehappened.

That is, in my view, completely wrong."

But he would not criticise the police for undertaking the review.

"It is not the police I blame for that, but the phone call should never have taken place."

He said it would be ''foolish" of him to start commenting on whether the police should have acted on Mr Williamson's call.

It was a matter for Police Commissioner Mike Bush to look at what happened and why.

Commenting on the release of documents yesterday under the Official Information Act on Judith Collins' dealings with the Oravida company, of which her husband is a director, Mr Key said there was nothing in the papers that was inconsistent with what Ms Collins had said publicly.

The papers showed that she always believed the dinner she had with its principals in Beijing was to be a private dinner.

Mr Key said he was alerted about three weeks ago to the fact that Oravida had made a $30,000 last year to the National Party that was soon to be disclosed.

Asked about whether the controversy around donations would revive calls for state funding of political parties, he said a lot of members of the public would not want to pick up the tab of election campaigns.

There would be all sorts of rows over the allocation of funding as there were over the broadcasting allocations for television and radio advertising.

National was not the only party that took donations from companies and individuals.

The Greens, for example, took $65,000 from Auckland businessman Phillip Mills.

"I'm sure they have met with him. They have asked questions in Parliament in recent times about green growth - an issue he cares a lot about."

"These things just aren't unique to National."

Before resigning on Thursday, Mr Williamson had been due to run a session at the conference on the 2013 Census.